Explanation:

"Percent" literally means "per #100#". If we say something is #18%#, that is the same as writing #18/100#. When we say #18%# of a number, it is similar to how we might say #3# of a number. In the latter case, we would multiply by #3# to represent it algebraically. In this case, we multiply by #18% = 18/100#.